So now I am thinking I'll build another and I am wondering how many inputs the TL084 will really take. There is a note on the schematic that says "Additional Input Channels if desired.", but I assume there must be a functional limit.

The number of inputs really shouldn't matter too much; at least not for a realistic number of inputs. I don't think you can really do any harm to the op-amp itself by having too many inputs. I imagine you'd get some signal degradation when you start summing a lot of inputs.

You should have no problem with 8 channels. I'm using Ken Stone's CGS04 DC Mixer, which only has one mono out with no stereo headphone circuitry. The schematics on their web page shows the circuit with 6 inputs, but if I'm not mistaken, the newer PCBs actually had 8 inputs. Anyway, it uses a single TL072.

I've migrated over from the Muffwiggler's, so I may look familiar to the double-members among us. But the modular has sucked me fully into SDIY and it was time to get an account here. It's great to see Kansas so well-represented in this community... who'd have figured?

The mixer already has input capacitors so you don't have to worry DC bias from the inputs.

The worst that can happen is that you'll get clipping. If you reduce the value for the input stage's negative feedback resistor (the 39K resistor) that will reduce the gain of the input stage and reduce clipping.

If you do this, you may want/need a corresponding increase for the value of the negative feedback resistor in the output stages (the 100K resistors connected in series with the 22pf capacitors).

Interestingly -- the line output for this mixer inverts the inputs -- and doesn't include a stage to invert the output from the first stage.

If you aren't using headphones, you may want to use one of the headphone stages to drive the line out -- that way you will output a non-inverted signal and you can adjust the overall gain with it's feedback resistor._________________-- Kevinhttp://kevinkissinger.com

Shouldn't be a problem. I built one similar to this with 6 inputs that ran on just one 9V cell, and that's fine. The only restriction is that you won't be able to get signals quite as loud from it, but as it is already way above what most audio circuits need that shouldn't be a problem._________________Stephen
www.Rainsbury.Net

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